1. Field of the Invention
This invention relates to digital output buffers and, more particularly, to digital output buffers with a control system that automatically adjusts the impedance compensation of the buffers.
2. Background Information
Digital data is communicated between integrated circuits over traces on printed circuit boards. These traces may be all on one printed circuit board or may be connected from a motherboard to a daughterboard. Such connections generally have lengths of several inches. When such connections are used to transmit data at higher speeds, such as speeds with a signaling rate of 100 MH or above, the trace behavior as a transmission line becomes increasingly important. To achieve reliable data transfer, the impedance of the output buffers must be compensated for the impedance of the transmission lines.
Output buffers are used to transmit data on the transmission lines. Differential line receivers are used to receive data from the transmission lines. Data is transmitted by transmitting a voltage that is higher or lower than a reference voltage. The differential line receivers compare the received voltage to the reference voltage to detect the transmitted data. Output buffers typically include a pair of driver circuits. One driver circuit pulls the transmission line to a voltage above the reference voltage to represent a first data state. The second driver circuit pulls the transmission line to a voltage below the reference voltage to represent a second state.
Output buffers may be programmably compensated for impedance by a driver circuit having two or more legs that can be independently enabled to provide programmable drive strength. The drive strength should be adjusted to compensate for the transmission line characteristics of the circuit board and also for variations in the circuit characteristics as fabricated. Accordingly, there is a need for output buffers that can automatically compensate for transmission line impedance in a variety of applications.